1. Field of the Invention
The present invention relates generally to a method of mineralizing air contaminants by oxidation, and more particularly, but not by way of limitation, to an improved method of removing contaminants from an air stream by passing the air stream through an ultra-low density, UV-accessible aerogel photocatalyst.
2. Brief Description of the Related Art
Environmental studies of the deleterious effects of air contaminants are pushing the clean air issue to the forefront of governmental legislation. The direct result of this is that the ability to remove air pollutants quickly, safely, and economically is now a recognized goal of many governmental and industrial organizations. Over the last three decades, in response to this recognized need, the scientific community has examined many different and quite novel remediation technologies as cost efficient candidates for maintaining clean air environments. Of these attempts, it has been the use of solar radiation of photocatalysts that has been seen as one of the most promising candidates for remediation of air contaminants. This perceived viability of photocatalysis for air remediation is due to its ability to catalyze the complete destruction of almost any hydrocarbon-based molecule ranging from common everyday solvents, odors, fragrances, proteins, mildew, viruses, bacteria and other organic vapors. This perspective has resulted in photocatalysis being investigated from many different perspectives: from identifying the most promising catalytic substrate to elucidating its oxidation mechanism. It has been found that titanium oxide and in particular its anatase crystal form is the most robust and catalytically active photocatalyst tested. As a result, efforts to commercialize titanium oxide photocatalytic processes have long been ongoing with some systems already being put in place.
The reality of photocatalysis is that it has stayed at the edge of being widely used by industry despite the fact that the energy driving photocatalysis is freely available solar radiation (or even that of an ultraviolet lamp) as well as the fact that complete destruction of even the most toxic organic compounds can be catalyzed. The primary factor slowing commercialization is seen as the need to increase the effectiveness of the photocatalysts.
Yet, increasingly, environmental concerns are overriding the wait for improvements in photocatalysts and are themselves becoming the driving force behind increasing implementation of photocatalytic processes "as is" in industry. Today, many major hotels are competitively advertising environmentally clean environments such as high quality air, nontoxic carpets, less polluting wall papers and other paraphernalia as well as financing improved air filtration and scrubbing systems. Many air conditioning vendors are experiencing requests for room systems that not only cool the air but also remove the smell of smoke and other odors or solvents. New products are thus being proposed that will attempt to clean the air we breathe and, ultimately, increase the competitiveness of the U.S. air conditioning industry by integrating photocatalytic air cleaners into commercial air conditioning systems. Such a system may be seen as analogous to the catalytic converter on an automobile, yet different in that not heat but light catalyzes the burning of organic pollutants. An added advantage is that photocatalysts also clean indoor air at ambient temperatures, allowing building managers to cut heating, ventilation and air conditioning costs by reducing the rate of air exchange or venting. Despite the advantages of photocatalysts, there are several significant problems which have limited the development of photocatalytic technologies for air decontamination.
First, known photocatalytic materials display a limited surface area to incident light. That is, for photocatalysis to occur, the photocatalytic surface must be accessible to both incident light and to the gases to be reacted. For any catalyst to be efficient, large active surface areas are desirable. Unfortunately, in high surface materials, including titanium oxide, internal pores usually account for nearly all the surface area. While these pores are accessible to gases via gas diffusion, they are not easily irradiated with incident light. It has been found that relevant UV wavelengths penetrate only approximately 4.5 .mu.m deep in titania. Thus, only pore surface area relatively near the external surface becomes photoactivated which results in low activity.
Second, photocatalysis may be limited by the competition of organic (contaminant) molecules for active surface sites. Particularly with very small pores (typical of high surface area materials), gas diffision, and adsorption may be highly restricted and become the limiting step in the oxidation process, thus again decreasing the usefulness of the catalyst.
Therefore, a need exist for an improved method of oxidizing a air contaminants with a photocatalyst that is substantially transparent and has an ultra low density which permits most of its high surface area to be jointly accessible to both incident radiation and to the air contaminants. It is to such a method that the present invention is directed.